1. Field of the Invention
The present invention relates to a method for perforating a heat-sensitive stencil sheet, and more specifically relates to a method of perforating a heat-sensitive stencil sheet by exposing it to a visible or infrared ray to make a master for stencil or screen printing.
2. Description of Related Art
As a structure of conventional heat-sensitive stencil sheets, is known a multilayer which is composed of a thermoplastic film laminated to an ink-permeable porous substrate made of Japanese paper or the like, or one layer which is composed simply of a thermoplastic film.
Methods for perforating such heat-sensitive stencil sheets to obtain masters for stencil or screen printing, include (1) a process of overlaying a heat-sensitive stencil sheet on images or letters that have been formed with carbon-containing materials such as pencils and toner by hand-writing or photocopying, and then exposing it to light from flash lamps, infrared lamps or the like to cause the portions of letters or images to emit heat so that the thermoplastic film of the stencil sheet is molten and perforated at portions that contact the images or letters, and (2) a process of melting and perforating the thermoplastic film of the stencil sheet by bringing the stencil sheet into contact with a thermal printing head which emits heat in dot-matrix forms so as to reproduce images in accordance with image data of electric signals that original images or letters have been transformed into.
In the above process (1), however, failure in perforation often occurs due to insufficient contact of the thermoplastic film of the stencil sheet with the original or the photocopied image portions of toner from which heat is emitted, or problems on so-called "pin holes" also occur which are phenomena of perforations caused in the stencil sheet at undesired portions by heat emitted from dust on the surface of the original or toner scattered out of the image portions. In the above process (2), there often occur perforation failure, conveying failure and wrinkling of the stencil sheet due to unevenness of pressure exerted to press the stencil sheet to the thermal printing head.
In order to solve such problems, the present inventor suggested, in Japanese Patent Application No. 284610/95, a method for perforating a heat-sensitive stencil sheet, which comprises ejecting a photothermal conversion material contained in a liquid from a liquid-ejecting means to transfer it together with the liquid to a heat-sensitive stencil sheet, and then exposing the heat-sensitive stencil sheet to a visible or infrared ray to perforate the heat-sensitive stencil sheet specifically at portions to which the photothermal conversion material has been transferred.
This perforating method comprises a first step of controlling a liquid-ejecting means to eject the liquid containing the photothermal conversion material as droplets onto a heat-sensitive stencil sheet in accordance with image data that have previously been transformed into electric signals while the liquid-ejecting means, which is maintained out of contact with the stencil sheet, is moved relative to the heat-sensitive stencil sheet, whereby the image is reproduced on the heat-sensitive stencil sheet as adherends in the form of dots composed of the photothermal conversion material and a second step of perforating the heat-sensitive stencil sheet specifically at sites to which the photothermal conversion material has been transferred, by subjecting the stencil sheet to a visible or infrared ray.
The perforation method is advantageous in that little pin hole is formed in the stencil sheet since the stencil sheet does not have to be brought into contact with the original or the liquid ejecting means upon perforation. Similarly, since the stencil sheet is liberated from contact with the original or a thermal printing head that has been required in conventional perforating methods, any problem of perforation failure due to contact failure does not occur, and the stencil sheet is perforated faithfully to image information.
However, according to the investigation conducted by the present inventor, it has been found that the perforantions in the stencil sheet made by the above methods include those which are continuous and those which are independent from each other. In the case of perforantions being in continuous state, ink passes through the stencil sheet in an amount more than needed to cause blur of printed images or letters, resulting in deterioration of print quality, and, furthermore, setting off or seeping through of the ink is apt to occur.
On the other hand, it has been found that in the case of perforations being independent from each other, a proper amount of ink passes through the perforated portion of the stencil sheet and, as a result, clear images and letters can be obtained upon printing and setting off hardly occurs, resulting in prints of high quality. However, it has been found that when the pitch between adjacent dots is too great as compared with the diameter of dots, then density of prints decreases or resolution deteriorates.
The object of the present invention is to provide a method of perforating a heat-sensitive stencil sheet, which is capable of providing a print of high quality that is sharp and high in density and free from setting off of ink, by forming perforations in the form of dots that are substantially independent from each other.